Vitamin A deficiency increases susceptibility to some types of infections, and there is currently an extensive literature regarding vitamin A deficiency and infection in experimental animal models, as can be found in reviews elsewhere (440,474-477).
Vitamin A supplementation or fortification has been shown to reduce the morbidity and mortality of diarrheal diseases among preschool children in developing countries. The reduction in diarrheal disease mortality appears to account for most of the reduction in overall mortality when vitamin A is given through fortification or supplementation on a community level. The main causes of diarrheal diseases among children in developing countries are rotavirus, Escherichia coli, and Shigella, Vibrio cholerae, Salmonella, and Entamoeba histolytica. The epidemiology, clinical features, immunology, and pathogenesis of diarrhea may differ according to characteristics of the pathogen, such as production of toxins, tissue invasion, fluid and electrolyte loss, and location of infection. In general, host defenses in the gut include gastric acidity, the presence of normal microflora, gut motility, mucus production, integrity of microvilli, local secretion of antibody, and cellmediated immunity, and vitamin A deficiency may impair some of these host defenses.
Large community-based clinical trials of vitamin A supplementation in Tamil Nadu, Nepal, and Ghana show that vitamin A supplemention reduced mortality from diarrheal disease but not pneumonia in preschool children (478,479). Vitamin A may reduce the morbidity of diarrheal disease through restoration of gut integrity (480) and enhancement of immune function (440,477). Urinary losses of vitamin A during diarrhea may be substantial in some children (481,482), and persistent diarrhea may reduce the bioavailability of vitamin A (483). Vitamin A supplementation (60 mg retinol equivalent [RE]) reduced morbidity in children with acute shigellosis (484) but the effects of vitamin A supplementation on other specific diarrheal pathogens has not been completely clarified.
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